This invention relates to methods for determining the permeability of a subsurface earth formation traversed by a borehole.
The permeability of an earth formation containing valuable resources such as liquid or gaseous hydrocarbons is a parameter of major significance to the economic production of that resource. These resources can be located by borehole logging to measure for example the resistivity and porosity of the formation in the vicinity of a borehole traversing the formation. Such measurements enable porous zones to be identified and their water saturation (percentage of pore space occupied by water) to be estimated. A value of water saturation significantly less than unity is taken as being indicative of the presence of hydrocarbons, and may also be used to estimate their quantity. However, this information alone is not necessarily adequate for a decision on whether the hydrocarbons are economically producible. The pore spaces containing the hydrocarbons may be isolated or only slightly interconnected, in which case the hydrocarbons will be unable to flow through the formation to the borehole. The ease with which fluids can flow through the formation, or permeability, should preferably exceed some threshold value to assure the economic feasibility of turning the borehole into a producing well. The threshold value may vary depending on such characteristics as the viscosity (in the case of oil): for example a highly viscous oil will not flow easily in low permeability conditions and if water injection is to be used to promote production there may be a risk of premature water breakthrough at the producing well.
The permeability of a formation is not necessarily isotropic. In particular, the permeability for fluid flow in a generally horizontal direction may be different from (and typically greater than) the value for flow in a generally vertical direction. This may arise for example from the effects of interfaces between adjacent layers making up a formation, or from anisotropic orientation of formation particles such as sand grains. Where there is a strong degree of permeability anisotropy it is important to distinguish the presence and degree of the anisotropy, to avoid using a value dominated by the permeability in only one direction as a misleading indication of the permeability in all directions.
Present techniques for evaluating formation permeability by borehole logging are somewhat limited. One tool that has gained commercial acceptance provides for repeat formation testing and is described for example in U.S. Pat. Nos. 3,780,575 to Urbanosky and 3,952,588 to Whitten, both assigned to the assignee of the present application. This tool includes the capability for repeatedly taking two successive samples at different flowrates from a formation via a probe inserted into a borehole wall. The fluid pressure is monitored and recorded throughout the sample extraction period and for a period of time thereafter. Analysis of the pressure variations with time during the sample extractions (draw-down) and the subsequent return to initial conditions (build-up) enables a value for formation permeability to be derived both for the draw-down and build-up phases of operation - see `RFT Essentials of pressure test interpretation` by Schlumberger, 1981.
However, the analysis assumes a homogeneous formation, and yields a single, `spherical` permeability value. Only in some cases can the analysis yield separate values for horizontal and vertical permeabilities, and then only with the incorporation of data from other logging tools or from core analysis. Up to the present it has been assumed that it is not possible to derive separate horizontal and vertical permeability values solely from the measurements provided by the single probe type of tool described in the above-mentioned U.S. patents. Furthermore, it is frequently found that the two values of spherical permeability obtained from the draw-down and the build-up measurements may differ by an order of magnitude. This leads to uncertainty as to which value, if either, should be taken as representative of the formation permeability for purposes of production evaluation.
It is an object of this invention to provide a more accurate method of determining permeability of earth formations by analysis of formation flow tests.
It is another object of this invention to provide a method of determining horizontal and/or vertical permeability of earth formations by analysis of formation flow tests.